Add my name to the masses asking for pass through on the HDMI ARC. Right now I have an Onkyo TX-NR708 receiver (circa 2010). It doesn't support 4K so the only way for me to connect my LG U875 UHD blu-ray player is to connect it directly to the LG OLED65B7A TV then use the ARC to get audio to the receiver. Currently, I'm only getting DTS to the receiver when the source is DTS HD Master Audio and the only options for the ARC are Auto and PCM. A passthrough option would really solve my problem it seems. Please update the firmware to allow for a passthrough.
In the mean time, I'm going to try an HDMI active splitter 1 in to 2 out with EDID settings, SIIG CE-H22K12-S1, and see if I can work around the problem.